Do not allow updatable apps without min_sdk_version.

All updatable modules are expected to declare their earliest platform
version they support.

Bug: 153539598
Test: m
Change-Id: I6243d276e5ab25a1007187ad34789ca1b4cc87bf

java/app.go

@@ -112,7 +112,9 @@ type appProperties struct {
 	IsCoverageVariant bool `blueprint:"mutated"`
 
 	// Whether this app is considered mainline updatable or not. When set to true, this will enforce
-	// additional rules for making sure that the APK is truly updatable. Default is false.
+	// additional rules to make sure an app can safely be updated. Default is false.
+	// Prefer using other specific properties if build behaviour must be changed; avoid using this
+	// flag for anything but neverallow rules (unless the behaviour change is invisible to owners).
 	Updatable *bool
 }
 
@@ -262,6 +264,9 @@ func (a *AndroidApp) checkAppSdkVersions(ctx android.ModuleContext) {
 		if !a.sdkVersion().stable() {
 			ctx.PropertyErrorf("sdk_version", "Updatable apps must use stable SDKs, found %v", a.sdkVersion())
 		}
+		if String(a.deviceProperties.Min_sdk_version) == "" {
+			ctx.PropertyErrorf("updatable", "updatable apps must set min_sdk_version.")
+		}
 	}
 
 	a.checkPlatformAPI(ctx)